There has been a continuing effort to prepare paper with improved physical properties. In particular, "cloth-like" properties of softness, absorbency, (particularly of aqueous fluids) and strength, particularly strength when wet. Softness is the tactile sensation perceived when the consumer holds the product, rubs it across the skin, or crumples it with the hand. This tactile sensation can be related to the stiffness of the paper product. Absorbency is a measure of the ability of the product to absorb quantities of liquid, particularly aqueous fluids or dispersions. Strength is the ability of the product to maintain physical integrity, and to resist tearing, bursting and shredding under use conditions, particularly when wet. Research and development efforts have been directed to improvement of each of these properties without adversely affecting the others, as well as improvement of two or three of these properties simultaneously.
Water-soluble cationic resins are often used as wet-strength additives in paper making. One such group of wet-strength additives are the polyamideepichlorohydrin resins sold under the trade name Kymene. See, for example, U.S. Pat. No. 3,700,623 to Keim issued Oct. 24, 1972; and U.S. Pat. No. 3,772,076 to Keim, issued Nov. 13, 1973. Another group of water-soluble cationic wet-strength resins are the polyacrylamides sold under the trade name Parez See, for example, U.S. Pat. No. 3,556,932 to Coscia et al, issued Jan. 19, 1971; and U.S. Pat. No. 3,556,933 to Williams et al issued Jan. 19, 1971.
The cellulosic fibers used in papermaking are negatively charged. Since the water-soluble wet-strength resins are cationic (positively charged), they are deposited and retained well when directly added to the aqueous pulp slurry. Such "wet-end addition" is highly desirable in papermaking. Subsequently in the papermaking process, these resins cross-link and eventually become insoluble in water. When this occurs, the wet-strength resin acts as a "glue" to hold the fibers of the paper together. This results in the desired wet-strength property.
Paper products made with such resins generally have a stiff, paper-like feel. To impart greater softness to the paper product, soft rubbery latexes can be used as the binder system. However, these latexes are either nonionic in character or else are partially anionic due to inclusion of anionic comonomers or surfactants. The nonionic latexes cannot be used as "wet-end additives" in a conventional papermaking process. Instead, these nonionic latexes have to be impregnated or pattern-printed on the subsequently laid paper furnish, such as by the process described in European Patent Application 33,988 to Graves et al, published Aug. 19, 1981.
An anionic latex can be used in a conventional wet-end additive papermaking process by adding a cationic polyelectrolyte. See, for example, U.S. Pat. No. 4,121,966 to Amano et al, issued Oct. 24, 1978; and U.S. Pat. No. 2,745,744 to Weidner et al, issued May 15, 1956. The cationic polyelectrolyte used is typically a water-soluble cationic wet-strength resin. Basically, the cationic polyelectrolyte, when added, destabilizes the dispersed anionic latex particles which then flocculate and deposit on the paper fibers. Accordingly, the cationic polyelectrolyte and anionic styrene-butadiene latex cannot be combined together until the point at which they are used as the binder system in papermaking.
Latexes have also been modified to provide cationic groups chemically bound on the surface of the latex particles. See, for example, U.S. Pat. No. 4,189,345 to Foster et al, issued Feb. 19, 1980; and U.S. Pat. No. 3,926,890 to Huang et al, issued Dec. 16, 1975. Incorporation of the cationic groups on the surface of the latex particles converts the latex into a wet-end additive like the water-soluble cationic wet-strength resins. These cationic latexes appear to have adequate colloidal stability, especially when nonionic or preferably cationic surfactants are added. However, the deposition and retention of the cationic latex particles on the paper fibers does not appear to be very great. Indeed, the cationic latex of the Foster et al patent appears to require a co-additive to enhance the deposition of the latex particles on the paper fibers.